A variety of medical implant devices are designed to be controlled, or modified, by the presence of strong magnetic fields. For example, pacemakers, defibrillators, cerebrospinal shunts, and vagus nerve stimulators are all designed to be controlled or modified by locally applied strong magnetic fields. Persons fitted with such implant devices are warned that exposure to magnetic fields above a certain level can cause unwanted modification of the implant device's function. Such warnings from device manufacturers range from 90 gauss for Medtronic Inc. of Minneapolis, Minn. “Strata” shunt to as little as 5 to 10 gauss for typical pacemakers and defibrillators.
Without a suitable magnetic survey meter, however, a patent fitted with such an implant device has no sense of how strong a particular magnetic field is and, therefore, is best advised to avoid all magnets or magnetic fields. This is not a simple task as such a patent faces potential hazards from an increasing variety of permanent magnets. For instance, cabinet door latches, screwdriver bit holders, electric motors, computer disk drives, badge holders, and audio speakers are just some of the places that permanent magnets having a flux density in excess of 5 gauss may be found. In addition, high-fidelity loudspeakers, audio speakers in telephones, cell phones, head phones and in toys that emit realistic sounds may also have magnetic fields that may pose a problem to such patients. Moreover, the threat is not always obvious. Magnets may also be located in what may appear to the ordinary person to be unlikely places. For instance, beneath some automobile seats there are magnets having flux densities in excess of 5 gauss that close a switch to verify that the seat is occupied. Even a sushi bar conveyor belt may contain magnets that are a problem for persons having a medical implant device.
Conventional magnetometers tend to be expensive, non-portable, and require electric power, making them of limited utility to a patent with a medical implant.
Even with an accurate magnetometer, the results require careful interpretation.
For instance, a major shortcoming of the existing warning system is an incorrect interpretation of the underlying physics involved in the activation of the magnetic switches. Almost all implant manufacturers express their warnings in terms units of “gauss.” The gauss (or the Tesla, which is 10,000 gauss) is defined as a magnetic flux density. No mention of a length scale is involved. A magnetic flux density in gauss is roughly the equivalent of saying something is red. It could be a red speck in the eye of a fly, the side of a barn, or Jupiter's Great Red Spot. All are red, but there is no discussion of how much red paint would be needed to give it a second coat. Magnetic flux densities of 1000 to 2000 gauss exist on a small scale in magnetic tapes and credit cards, but because of the tiny size of their magnetic domains, they present no hazard to a medical implant. Similarly, many scientific equipment retailers such as, but not limited to, Radio Shack Corporation of Fort Worth, Tex., sells rare-earth magnets with 10,800 gauss flux densities but the small size (1.5 millimeter thick and 5 millimeter diameter) renders the magnet incapable of reaching a 90 gauss level at a typical depth inside a patient.